Serde: How to parse JSON partially. 7 years later?

help
1

(This was asked back in 2016. Back then the answer was that it didn't work yet. Progress?)

I'd like to parse structures which contain substructures of unknown form. I can't write

   struct Item {
       name: String,
       value: JsonValue
   }

   struct Msg {
       items: Vec<Item>
   }

because JsonValue is not accepted by serde as a deseralization target. I'd like to get the content parsed into the usual JsonValue tree, from which I can then extract fields.

On a related note, is it possible to deserialize key-value pairs into an array of enums?

2

serde_json::Value works in that way and it has existed since the initial release of serde to the crates.io at 2015.

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3

There's probably a simpler way, but you can always write your own implementation of Deserialize instead of deriving it: there's a bit of busy work with implementing a serde::de:: Visitor because serde needs to handle formats that aren't self-describing. For JSON, just use deserialize_any in the Deserialize impl, passing a Visitor impl with a visit_map() where you can read and switch on the keys and values in order.

4

You may also want to look into the optional raw_value feature, which lets you defer parsing part of the JSON tree until later.

5

Which public visible project has an example of such partial parse?

6

Within the serde architecture it's actually harder to not allow partial parse. It it bans partial parse it should document why it does so with extra effort. For example serde_urlencoded crate doesn't allow nested fields as documented, since x-www-form-urlencoded format doesn't have such concept.

1 Like
7

How do you do the second part of this? I have have a JsonValue, and want to parse that into a struct for which Deserialize has been derived. There's "from_str", but not "from_value". I suppose I could serialize the JsonValue into a string and then deserialize from the string, but that's silly.

8

There literally is "from_value". from_value in serde_json - Rust

3 Likes
9

Ah, thanks.